This invention relates to a process for heating particle-filled adhesive compositions with the aid of an electrical, magnetic or electromagnetic alternating field. The present invention also relates to a process for bonding metallic and nonmetallic materials by heating particle-filled adhesive compositions with the aid of the alternating fields mentioned above.
In many branches of industry, particularly in the metal-processing industry, for example the motor industry, in the manufacture of utility vehicles and the associated supplier industries or even in the production of machines and domestic appliances and in the building industry, identical or different, metallic and nonmetallic substrates are being increasingly joined together by adhesives or sealants. This method of joining structural components is increasingly replacing conventional joining techniques, such as riveting, screwing or welding, because bonding/sealing offers a number of technological advantages. This is particularly the case in the joining of materials differing in composition, as in the joining of metals to plastics and/or composites or even in the joining of different metals, for example steel parts to parts of aluminium or magnesium.
One requirement which modern production processes in the industrial sector, more particularly in car manufacture, are expected to satisfy is that the various joined parts should lend themselves to rapid subsequent processing. A basic disadvantage of reactive adhesive systems is that a relatively long period of time is required for the curing of the adhesives and hence for the buildup of strength. Although this basic disadvantage is not so pronounced in the case of hotmelt adhesives, non-reactive (non-postcrosslinking) hotmelt adhesives can only be exposed to high temperatures over a limited range, particularly where high-strength adhesive bonds are involved. With reactive adhesives, the curing process is normally accelerated to a very considerable extent by heating so that strength is built up very quickly. To this end, the entire workpiece already provided with adhesive and joined is normally heated. This can be done in various ways, for example in large ovens, by hot air blowers, by direct exposure of the workpieces or those areas of the workpieces which are to be bonded to flames or by exposure to IR heaters. Heat-sensitive workpieces cannot be treated in this way so that there is a need to provide a process in which only the adhesive itself is heated. Such selective heating of the adhesive is known in principle. Thus, WO 99/09712 describes a process for the at least partial curing of sealants and adhesives, particularly in the direct glazing of motor vehicles, by exposing at least part of the sealant and adhesive to microwave energy.
The use of microwave energy for heating electrically nonconductive materials is known per se. A comprehensive account of this technology can be found in R. V. Decahau and R. A. Peterson's “Microwave Processing and Engineering”, VCH Verlagsgesellschaft, 1986.
The use of microwaves for curing polyurethane systems such as occur in adhesive compositions is also known in principle (U.S. Pat. No. 4,083,901). In these known processes for utilizing microwave energy, however, the substrates to be heated are always exposed to the microwave field in large closed chambers, such as for example large ovens or belt dryers. Unfortunately, such processes cannot be applied where bonds and seals are to be formed on large and difficult items, such as for example car parts or entire care bodies, and only very small areas in relation to the overall size of the part are to be exposed to microwave energy.
The amount of microwave energy to be supplied for the partial or complete curing process depends on various factors, for example the viscosity of the sealant and adhesive used and the thickness of the layer to be cured, the amount supplied being greater, the higher the viscosity of the sealant and adhesive and the smaller the layer thickness.
To solve these problems, WO 88/09712 proposes applying the microwave energy in pulses using specially designed emitters, a first group of pulses in which the pulse amplitudes decrease being delivered. This pulse-like application of the microwave energy initially delivers a relatively large but brief supply of energy so that part of the sealant and adhesive is heated to a considerable extent without any sign of burning or decomposition. Between the delivery of the first microwave pulse and the delivery of the following pulse of a second group, temperature equalization occurs in the sealant and adhesive through thermal conduction so that the following pulse does not cause any overheating of the, initially, relatively intensively heated part of the sealant and adhesive. By virtue of the heating effect of the first pulse and the resulting increase in the temperature of the sealant and adhesive, a smaller amount of energy is then delivered because the sealant and adhesive can now be exposed to microwave pulses of decreasing amplitude. In practice, however, this process has shown that controlling the amount of energy is difficult and is limited where extremely short cycle times are available for the joining process and hence for curing.
EP-A 545 033 describes a process for joining an electrical winding to an iron core, more particularly of ignition coils, using an adhesive curable by heat. In this process, the windings and core are exposed to a high-frequency alternating field so that they are heated. In this way, the unit formed by the winding and core is rapidly and uniformly heated so that the adhesive is cured. However, this process can only be applied to the structural aspects of special electrical components.
EP-A-237 657 describes a method for joining a strip of carpet. To this end, the adhesive layer is said to contain a high-frequency induction powder or an adhesive tape is said to contain an electrically conductive metal foil. It is proposed to introduce powder-form iron, cobalt, nickel, aluminium, carbon or graphite as conductive or inductive materials into a heat-sensitive material. These powder-form particles preferably have a flat lamellar structure in order to accelerate heating by induction heating.
U.S. Pat. No. 5,833,795 describes a method for repairing composite materials by bonding a repair patch to the product consisting of a composite material. It is proposed to use an epoxy adhesive containing magnetic particles so that the adhesive or the epoxy resin can be cured by electromagnetic excitation of the magnetic particles. More specifically, microwave curing is proposed. To this end, the magnetic particles, such as iron silicide, should be present in quantities of about 15 to 20% by volume and the magnetic particles should have a Curie temperature in the temperature range necessary for curing.
WO 98/05726 describes a process for adhesively joining rubber components. In this process, the surfaces of the rubber parts to be joined are placed beside a special device. This device contains a target element which absorbs electromagnetic waves and which is in contact with a heat-activatable adhesive. The electromagnetic waves heat the target element to active the adhesive. The surfaces to be joined are held together and the device is exposed to electromagnetic radiation so that sufficient heat is produced to activate the adhesive and establish a bond between the rubber parts and the working surface. A very similar device is described in WO 98/05728. According to this document, the device in question is particularly efficient when used between components that are transparent to electromagnetic waves.
WO 98/51476 describes a process for forming a system for carrying liquids and, more particularly, a process for joining a rigid thermoplastic vessel to a flexible thermoplastic or thermoset, elastomeric line which is produced using an electromagnetic joining technique. The flexible line is designed in such a way that it guarantees an exact fit onto the rigid vessel under compressive forces. To this end, an electromagnetically setting adhesive is applied between the components before the electromagnetic forces are applied. The adhesive material is said to be a thermoplastic material which contains at least 60% by weight of uniformly distributed particles absorbing electromagnetic energy.
Accordingly, although it is known from the prior art that the adhesive can be exposed to electromagnetic radiation in two ways: first, devices or substrates adjacent the adhesive layer are heated by electromagnetic radiation and, second, particles absorbing electromagnetic radiation are added to the adhesive and have to be dispersed in very high percentages into the adhesive matrix, such high percentages of metal powders or other particles absorbing electromagnetic radiation often have a very adverse effect on the strength of the adhesive thus cured, particularly in the case of high-strength adhesives.
Against the background of this prior art, the problem addressed by the present invention was to provide processes which could convert the electromagnetic radiation more effectively into the heat required for heating the adhesive and which would not adversely affect the properties of the adhesive.